Hydrogen sulfide is characterized by a well-known "rotten egg" odor and is prevalent at most wastewater treatment plants. Although hydrogen sulfide can be fatal at high concentrations in the gaseous phase, the need for treatment is generally governed by the objectionable odor. Aqueous phase hydrogen sulfide is present in several areas of the United States, especially parts of Florida and California. Aqueous phase hydrogen sulfide can cause an odor problem depending on the pH of the water, however, the major concern is the objectionable taste imparted on the water by the dissolved hydrogen sulfide. Activated carbon has been known to remove hydrogen sulfide from both gaseous and aqueous phases through a catalytic oxidation process. The reaction rate of the catalytic oxidation process has generally been too slow to be commercially viable, therefore, the use of chemical impregnants added to the activated carbon or chemical addition to the gaseous or aqueous streams was necessary.
The use of activated carbon impregnated with caustic compounds such as sodium hydroxide and potassium hydroxide has been practiced for many years. The use of the caustic impregnation increases the reaction rate of the hydrogen sulfide oxidation. The majority of the hydrogen sulfide is oxidized to elemental sulfur while a minor portion is converted to sulfuric acid. After a sodium hydroxide impregnated carbon has become exhausted and can no longer convert additional hydrogen sulfide, the carbon can be chemically regenerated with a 50% sodium hydroxide solution. This process, although commercially viable, results in the generation of a sulfur containing caustic waste, which must be properly disposed. Caustic impregnated materials are also known to be susceptible to uncontrolled thermal excursions resulting from a suppressed combustion temperature caused by the caustic impregnation.
Other impregnants such as potassium iodide have been used to increase the reaction rate of hydrogen sulfide oxidation. Although the use of potassium iodide increases the reaction rate, and reduces the potential for uncontrolled thermal excursions, the major reaction product is elemental sulfur. The formation of elemental sulfur significantly reduces the possibility of chemical regeneration due to the stability of the elemental sulfur in the carbon pore structure. The possibility of thermal reactivation is also substantially reduced due to the need to scrub reactivation off gases that would contain high concentrations of sulfur dioxide.
Addition of chemicals to gaseous streams has also been practiced commercially. The addition of ammonia to gas streams containing hydrogen sulfide has been shown to increase the reaction rate of the hydrogen sulfide oxidation, however, the resulting reaction product is overwhelmingly elemental sulfur, resulting in a one-time use of the activated carbon.
All of the prior art methods for improving the removal of hydrogen sulfide from gaseous streams have certain disadvantages, which make the processes unattractive from a commercial standpoint. Chief among these is an inability to determine in a rapid and convenient manner the suitability of a char for such applications prior to its use, in particular the intrinsic catalytic activity of the char for hydrogen sulfide conversion. As a result of this shortcoming, it is not possible to know or even to estimate during the preparation of a char the utility of the final product short of actual testing in the application itself. None of the measures of typical char properties, e.g. iodine number and apparent density, has ever shown a clear correlation with utility in these applications, although some are known to affect overall reaction rates, primarily as a result of mass transport effects. This can be seen more clearly when several chars possessing nearly identical physical properties are contacted with a given hydrogen sulfide-containing process stream, yet show significantly different rates of hydrogen sulfide conversion and removal.
Accordingly, it is the object of the present invention to provide an improved process for the catalytic chemical conversion and removal of hydrogen sulfide in gaseous and liquid media by contacting said media with a carbonaceous char in which the intrinsic catalytic activity of the char is measured and known prior to use. It is further the object of the present invention to use the intrinsic catalytic activity of the char measured by a rapid and simple test as an indication suitability of the char for the application of hydrogen sulfide conversion.